Surface metallurgical and tribological characteristics of TiN-coated bearing steels

Abstract

In this study, an investigation was made of the rolling contact fatigue (RCF) characteristics of an aerospace bearing steel 440C coated with TiN films of various thicknesses. Particular emphasis was placed on the wear and corrosion properties of the film-substrate system and how these properties correlate with the RCF behaviour. Two physical vapor deposition methods, magnetron sputtering and hollow cathode discharge ion plating, were used to evaluate the potential of each as TiN coating systems for advanced bearing applications. The surface metallurgical characteristics of the coatings were determined by employing surface and structure analytical techniques. RCF testing was utilized to evaluate the tribological characteristics of TiN-coated and uncoated specimens. The results showed that TiN coatings with thicknesses below 1 μm resulted in marked improvements in the RCF lives of base steel substrates under both mild and heavy loading conditions. Thick coatings tended to delaminate and chip when tested under high loads and shortened the fatigue life of bearing steels. However, thick coatings were also found to be effective in increasing the fatigue lives under low load conditions. On the basis of the experimental observations, optimum conditions leading to improved RCF properties were determined. A hypothetical model relating surface metallurgical characteristics of TiN coatings to the RCF performance of coating-substrate systems is suggested.